Method and device for applying electron-emissive coatings to coating-supports of indirectly heated thermionic cathodes of electric discharge tubes



: EM ISSIVE MATERIAL Feb. 6, 1962 H. H. BLANK ETAL 4 METHOD AND DEvIcE FOR APPLYING ELECTRON-EMISSIVE COATINGS TO COATING-SUPPORTS 0F INDIRECTLY HEATED THERMIONIC CATHODES OF ELECTRIC DISCHARGE TUBES Filed April 24, 1958 AIR 9 EMISSIVE MATERIAL INVENTOR H H.BL 8 PG. VAN ZANTEN A KUIPE F H. R.

United States atent Hans Hinrich Blank, Hamhurg-Wellingsbuttel, Germany,

and Pieter Geert van Zanten, Adrianna Kniper, and Friedrich Hermann Raymund Almer, Eintlhoven, Netherlands, assignors to North American Philips Company, Inn, New York, N.Y., a corporation of Bein- Ware Filed Apr. 24, 1958, Ser. No. 730,633 Claims. (Cl. 2d4--181) This invention relates to methods and devices which enable the coating supports of indirectly heated thermionic cathodes to be-coated with a smooth, compact emission paste layer of uniform thickness, firm adherence of the layer being ensured.

It is known to coat cathodes by spraying. Neglecting the fact that this process requires the use of copious amounts of emissive materials and solvents and that it provides a very poor definition of the length of the sprayed coating, it only permits of producing layers the surfaces of which have a comparatively high roughness 1 of to 40 microns.

Compared with this known method, the cataphoretic coating of cathode sleeves offers certain advantages, however this latter method also 'gives rise to difficulty with respect to a sharp definition of the length of the coat ing and, in addition, the inner space of the cathode sleeve must be sealed during the coating process.

Furthermore a method of coating tubular coating supports of thermionic cathodes with emission paste is known in which a wiper is wetted by emission paste and subsequently brought into contact with the cathode sleeve which revolves about its horizontally arranged axis, the emission paste provided on the Wiper being transferred to the cathode sleeve. In this method, the application of the emission paste can be effected simultaneously by wiping and by cataphoretic deposition. It is, however, difficult to produce uniform layers by means of the resilient wiper, and it has been found impossible to avoid the occurrence of a seam at the point of the coating surface at which the wiper leaves the applied coating on termination of the wiping process.

It is an object of the present invention to provide a method in which the above-mentioned disadvantages are avoided. According to the invention, in a method of applying the emissive coating to the coating support of indirectly heated thermionic cathodes of electric discharge tubes, the emission paste is conveyed by a rotating roller from the coating vessel to the likewise rotating coating support.

The coating support may revolve in known manner between two centres, preferably at a peripheral speed which is less than that of the roller, so that there is a constant a supply of emission paste.

When a sufficiently thick coating has been applied, the film of emission paste on the roller can be stripped off mechanically, for example by an air current, so that the formation of a seam on the coating of the layer support is avoided with certainty.

The method in accordance with the invention permits of effecting by simple means a cataphoretic deposition simultaneously with the application of the emission paste by means of a rolling process and to this end the coating roller must be at a positive potential with respect to the coating support. it also enables the necessary drying process to be performed in a very simple manner sub sequent to the coating process in that the coated support i ce continues to revolve, the drying period being about 20 seconds.

In order that the invention may readily be carried into effect, one embodiment thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawing.

A coating roller 1 having preferably backed-off edges, which in operation rotates in the direction of the inscribed arrow, dips in a coating vessel 2 so that it continuously carries along a film of emission paste. This film comes into contact with a coating support 4, which is spaced from the roller 1 by about 1.5 mms. and rotates in a sense opposite to that of the roller 1 and at a lower peripheral speed. ,The sense of rotation of the support 4 is also indicated by an arrow. The speed of rotation of' the coating support 4 can be from 10 to 15 revolutions per second.

From a supply container 8, which is provided with a stirring device 7 or the like, the emission paste is pumped by means of a circulation pump 6 into the coating vessel 2 from which it returns to the supply container 8 through an overflow pipe 9. Thus, the concentration of the coating material to be applied can be kept constant.

Adjacent the coating roller 1 provision is made of an air duct 5, through which on termination of the coating process an air stream is ejected which strips the film from the roller.

The roller 1 and the coating support 4 are connected in the same electric circuit which is closed when the film 12 of emission paste comes into contact with the support 4 at the beginning of the coating process. This ensures a cataphoretic deposition. The duration of the cataphoretic deposition is preferably controlled by an electronic timing device known per se, which is actuated by the contact between the coating support and the emission paste... Reference numeral 10 denotes a regulating resistance and reference numeral 11 an instrument for measuring the current which produces the cataphoretic deposition.

The invention permits of coating circular, oval and rectangular cathode profiles. The coating is very even since it consists of a plurality of filmy layers. It has been found that the roughness of the surface of the applied coatings is less than 1 micron.

The .composition of the emission paste is not critical, and the usual conventional materials can be used, that is, a mixture of alkaline earth carbonates added to a liquid containing a binder to form a paste. The speeds of rotation of the transfer roller and the support depend upon the viscosity of the paste mixture and the coating thickness desired. Adjustment of these variables for best results is relatively simple. The following example will illustrate one set of conditions for carrying out the invention, but it will be understood that variations are obviously possible within the scope of the invention.

The emission paste consisted of 1900 gr. of

(300 mesh) mixed with 1350 ccm. of nitrocellulose, ccm. of diacetonealcohol and 1500 ccm. of methanol. The viscosity of this mixture at 20 C. was 35 Ford cup A. With this viscosity mix, a roller diameter of 40 mm., a cathode sleeve or support diameter of 0.85 mm, and a spacing of 1.5 mms., a speed of rotation for the roller of 27 rpm. (clockwise) and for the support of 1000 r.p.m. (counterclockwise) was found satisfactory. The surface speed of the roller amounted to 3.4 m./min., and that of the support to 2.5 m./min. The transfer time was 1 second. When a cataphoretic technique is employed, which is preferred, the voltage and current is not critical; voltages'varying between 50 and 500 volts appiled between the transfer roller and the support may be used. For the above specific example, 150 volts and a current of about 2000 microamperes was used. With this example, a layer thickness of 20 microns was applied to the support. To obtain thicker layers in a single coating step, mixtures with a higher viscosity may be employed.

To terminate the coating process without leaving a seam on the support, the roller coating is air-stripped by directing a stream of air tangentially at the roller surface. For the specific example, the air duct 5 was spaced 250 microns from the roller surface, and an air pressure of 4 atmospheres (absolute) was used to strip the roller coating.

The coating on the support dried in air in about 20 seconds while the support continued to rotate.

While we have described our invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

This application is a continuation-in-part of application Serial No. 688,958, filed October 8, 1957, now abandoned.

What is claimed is:

1. A method of applying a coating of emissive material onto a support for making a cathode, comprising the steps of rotating the support in contact with a film of emissive material carried on a rotating coating roller thereby transferring emissive material onto the support, and terminating the material-transferring process by an air stripping process for removing the film of emissive material from the coating roller while it and the support continue to rotate. V

2. A method of applying a coating of emissive material onto a support for making a cathode, comprising the steps of rotating the support in contact with a film of emissive material carried on a rotating coating roller while maintaining a potential diiference between the support and the roller thereby cataphoretically transferring emissive material onto the support, terminating the material-transferring process by an air stripping process for removing the film of emissive material from the coating roller while it and the support continue to rotate, and drying the coating on the support while it rotates.

3. A method as set forth in claim 2 wherein the support is rotated in a direction opposite to that of the coating roller, and the surface velocity of the former is smaller than that of the latter.

4. A method of applying a coating of emissive material onto a support for making a cathode, comprising the steps of rotating a coating roller in contact with a supply of paste-like emissive material thereby forming a film of said material on the coating roller, rotating the cathode support in contact .with the emissive material film to transfer emissive material onto the support, air stripping the emissive material film from the coating roller to terminate the material-transferring process while the coating roller and support continue to rotate, and drying the coating on the support while it rotates.

5. A method of applying a coating of emissive material onto a support for making a cathode, comprising the steps of rotating a coating roller in one direction in contact with a supply of paste-like emissive material thereby forming a film of said material on the coating roller, rotating the cathode support in the opposite direction in con tact with the emissive material film to transfer emissive material onto the support, maintaining a potential difference between the cathode support and coating roller, with the roller positive, whereby cataphoretic deposition occurs, air-stripping the emissive material film from the coating roller to terminate the material-transferring process while the roller and support continue to rotate, and drying the coating on the support while it rotates.

6. A method as set forth in claim 5 wherein the emissive material coating on the support is dried while the latter continues to rotate.

7. Apparatus for coating a support with emissive material to form a cathode, comprising a vessel for holding a supply of paste-like emissive material, a coating roller whose surface contacts the supply of emissive material and means for rotating the coating roller whereby a film of emissive material is established on the roller, means for supporting and rotating the cathode support in contact with the film of emissive material on thesupport thereby to transfer material onto the support, and air stripping means located between the vessel and the support on the side of the roller travelling toward the support for removing the film of emissive material from the coating roller thereby to terminate the material-transferring operation.

8. Apparatus for coating a support with emissive material to form a cathode, comprising a vessel for holding a supply of paste-like emissive material, a coating roller whose surface contacts the supply of emissive material and means for rotating the coating roller in one direction whereby a film of emissive material is established on the roller, means for supporting and rotating the cathode support in the opposite direction in contact with the film of emissive material on'the support thereby to transfer material onto the support, means for maintaining the coating roller at a positive potential relative to the support so that cataphoretic deposition occurs, and means located between the vessel and the support on the side of the roller travelling toward the support for air stripping the film of emissive material from the coating roller thereby to terminate the material-transferring operation.

9. Apparatus as set forth in claim 8 wherein the lastnamed means comprises an air duct mounted so as to direct a stream of air tangentially at the roller surface.

10. Apparatus as set forth in claim 9 wherein means are provided for agitating and continuously circulating the supply of emissive material to maintain more uniform the composition of the said emissive material.

References tilted in the file of this patent UNITED STATES PATENTS 2,084,094 Kronquest June 15, 1937 2,161,187 Mueller Q June 6, 1939 2,172,326 Wittich Sept. 5, 1939 2,258,659 Mosler Oct. 14, 1941 2,363,817 Taylor Nov. 28, 1944 2,442,863 Schneider June 8, 1948 2,525,920 Mackey Oct. 17, 1950 2,653,566 Worden Sept. 29, 1953 2,679,231 Pamper May 25, 1954 2,688,306 Fahrni Sept. 7, 1954 

5. A METHOD OF APPLYING A COATING OF EMISSIVE MATERIAL ONTO A SUPPORT FOR MAKING A CATHODE, COMPRISING THE STEPS OF ROTATING A COATING ROLLER IN ONE DIRECTION IN CONTACT WITH A SUPPLY OF PASTE-LIKE EMISSIVE MATERIAL THEREBY FORMING A FLIM OF SAID MATERIAL ON THE COATING ROLLER, ROTAING THE CATHODE SUPPORT IN THE OPPOSITE DIRECTION IN CONTACT WITH THE EMISSIVE MATERIAL FLIM TO TRANSFER EMISSIVE MATERIAL ONTO THE SUPPORT, MAINTAINING A POTENTIAL DIFFERENCE BETWEEN THE CATHODE SUPPORT AND COATING ROLLER WITH THE ROLLER POSITIVE, WHEREBY CATAPHOTIC DEPOSITION OCCURS ARI-STRIPPING THE EMISSIVE MATERIAL FLIM FROM THE COATING ROLLER TO TERMINATE THE MATERIAL-TRANSFERRING PROCESS WHILE THE ROLLER AND SUPPORT CONTINUE TO ROTATE AND DRYING THE COATING ON THE SUPPORT WHILE IT ROTATES.
 8. APPARATUS FOR COATING A SUPPORT WITH EMISSIVE MATERIAL TO FORM A CATHODE COMPRISING A VESSEL FOR HOLDING A SUPPLY OF PASTE-LIKE EMISSIVE MATERIAL A COATING ROLLER WHOSE SURFACE CONTACTS THE SUPPLY OF EMISSIVE MATERIAL AND MEANS FOR ROTATING THE COATING ROLLER IN ONR DIRECTION WHEREBY A FLIM OF EMISSIVE MATERIAL IS ESTABLISHED ON THE ROLLER MEANS FOR SUPPORTING AND ROTAING THE CATHODE SUPPORT IN THE OPPOSITE DIRECTION IN CONTACT WITH THE FLIM OF EMISSIVE MATERIAL ON THE SUPPORT THEREBY OT TRANSFER MATERIAL ONTO THE SUPPORT, MEANS FOR MAINTAINING THE COATING ROLLER AT A POSITIVE POTENTIAL RELATIVE TO THE SUPPORT SO THAT CATAPHORETIC DEPOSITION OCCURS, AND MEANS LOCATED BETWEEN THE VESSEL AND THE SUPPORT ON THE SIDE OF THE ROLLER TRAVELLING TOWARD THE SUPPORT FOR AIR STRIPPING THE FLIM OF EMISSIVE MATERIAL FROM THE COATING ROLLER THEREBY TO TERMINATE THE MATERIAL-TRANSFERING OPERATION. 